Fast-neutron-induced fission of 238U at an energy just above the fission threshold is studied with a novel technique which involves the coupling of a high-efficiency γ-ray spectrometer (MINIBALL) to an inverse-kinematics neutron source (LICORNE) to extract charge yields of fission fragments via γ-γ coincidence spectroscopy. Experimental data and fission models are compared and found to be in reasonable agreement for many nuclei; however, significant discrepancies of up to 600% are observed, particularly for isotopes of Sn and Mo. This indicates that these models significantly overestimate the standard 1 fission mode and suggests that spherical shell effects in the nascent fission fragments are less important for low-energy fast-neutron-induced fission than for thermal neutron-induced fission. Finally, this has consequences for understanding and modeling the fission process, for experimental nuclear structure studies of the most neutron-rich nuclei, for future energy applications (e.g., Generation IV reactors which use fast-neutron spectra), and for the reactor antineutrino anomaly.

@article{osti_1378544,
title = {Anomalies in the Charge Yields of Fission Fragments from the U(n,f)238 Reaction},
author = {Wilson, J. N. and Lebois, M. and Qi, L. and Amador-Celdran, P. and Bleuel, D. and Briz, J. A. and Carroll, R. and Catford, W. and De Witte, H. and Doherty, D. T. and Eloirdi, R. and Georgiev, G. and Gottardo, A. and Goasduff, A. and Hadyńska-Klęk, K. and Hauschild, K. and Hess, H. and Ingeberg, V. and Konstantinopoulos, T. and Ljungvall, J. and Lopez-Martens, A. and Lorusso, G. and Lozeva, R. and Lutter, R. and Marini, P. and Matea, I. and Materna, T. and Mathieu, L. and Oberstedt, A. and Oberstedt, S. and Panebianco, S. and Podolyák, Zs. and Porta, A. and Regan, P. H. and Reiter, P. and Rezynkina, K. and Rose, S. J. and Sahin, E. and Seidlitz, M. and Serot, O. and Shearman, R. and Siebeck, B. and Siem, S. and Smith, A. G. and Tveten, G. M. and Verney, D. and Warr, N. and Zeiser, F. and Zielinska, M.},
abstractNote = {Fast-neutron-induced fission of 238U at an energy just above the fission threshold is studied with a novel technique which involves the coupling of a high-efficiency γ-ray spectrometer (MINIBALL) to an inverse-kinematics neutron source (LICORNE) to extract charge yields of fission fragments via γ-γ coincidence spectroscopy. Experimental data and fission models are compared and found to be in reasonable agreement for many nuclei; however, significant discrepancies of up to 600% are observed, particularly for isotopes of Sn and Mo. This indicates that these models significantly overestimate the standard 1 fission mode and suggests that spherical shell effects in the nascent fission fragments are less important for low-energy fast-neutron-induced fission than for thermal neutron-induced fission. Finally, this has consequences for understanding and modeling the fission process, for experimental nuclear structure studies of the most neutron-rich nuclei, for future energy applications (e.g., Generation IV reactors which use fast-neutron spectra), and for the reactor antineutrino anomaly.},
doi = {10.1103/PhysRevLett.118.222501},
journal = {Physical Review Letters},
number = 22,
volume = 118,
place = {United States},
year = {2017},
month = {6}
}

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